West Ham players confronted a group of AZ Alkmaar fans at the end of the teams' match on Thursday after the section of the stadium containing the Premier League side's friends and families came under attack.
The ugly scenes inside the AFAS Stadion in the Netherlands followed West Ham's 1-0 victory in the UEFA Conference League semifinal second leg, a win that secured the English club its first European final since 1976.
The West Ham players were celebrating their famous victory on the pitch when a group of AZ supporters started instigating trouble, with videos on social media showing the supporters, who were dressed in black, pulling down a fence and then running towards the stands above the dugouts, before throwing punches towards people in the stand.
Videos also showed a number of West Ham players leaping over the hoardings in an attempt to intervene before security arrived to calm the situation.
"We'll need to wait for the dust to settle to see what it is, but I think the biggest problem was the area where all the players had all their families in," West Ham manager David Moyes told BT Sport after the game.
"That is where the problem came, and a lot of players were getting angry because they couldn't get to see if they were OK."
Moyes said he had family and friends in that particular section and admitted he was worried when he first saw what was unfolding.
"What I don't want to do in any way is blight our night because it certainly wasn't West Ham supporters looking for trouble," he told reporters.
"Security wanted to take me inside, but I had to make sure my players were not going to get involved."
'Only a matter of time before someone is killed'
West Ham's goalscorer on the night Pablo Fornals also said he was worried when he saw the group of fans running towards the stands.
"I was really concerned about how the family of my teammates and the West Ham family are," he told Sky Sports.
"Hopefully, everyone is OK and the police can do their job and realize who did it."
Geoff Pearson, an expert in crowd behavior at soccer stadiums, wrote on Twitter that it was "complete failure" from a security point of view.
"Scenes we used to see regularly outside stadia on European matchdays are becoming increasingly common inside. It is only a matter of time before someone is killed," he said.
Local police told CNN Sport they would investigate footage of the incident and try to identify supporters involved. No arrests had yet been made, they added.
"Together with AZ, the municipality of Alkmaar and the public prosecution service we will evaluate last night's incidents, which we regret having happened. This kind of behaviour has no place in football," a police statement said.
CNN has reached out to West Ham and AZ Alkmaar for comment but had not received a response at time of publication.
UEFA, European football's governing body, told CNN it was too early to comment as official reports from all of Thursday's matches were still being received and reviewed.
According to Reuters, there was a similar incident after the first leg in London last week when AZ supporters were accosted by West Ham fans.
"What happened last week was very unfortunate and then you get the same feeling as what happened tonight," said AZ Alkmaar coach Pascal Jansen, according to the BBC.
"I feel a little bit ashamed it happened in our stadium, but you have to control your emotions."
